The process control industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter can provide an output related to the sensed process variable over a process control loop to a control room, such that the process can be monitored and controlled.
The process control loop can be a two-wire 4-2 mA process control loop. With such a process control loop, the energization levels are low enough that even under fault conditions the loop generally will not contain enough electrical energy to generate a spark. This is particularly advantageous in flammable environments. Process variable transmitters can sometimes operate on such low energy levels that they can receive all electrical power from the 4-20 mA loop. The control loop may also have digital signals superimposed on the two-wire loop according to a process industry standard protocol such as the HART.RTM. digital protocol.
Low Power Time Domain Reflectometry Radar (LPTDRR) instruments have been used recently to measure the level of process products (either liquids or solids) in storage vessels. In Time Domain Reflectometry, electromagnetic energy is transmitted from a source, and reflected at a discontinuity. The travel time of the received pulse is based on the media through which it travels. One type of LPTDRR is known as Micropower Impulse Radar (MIR), which was developed by Lawrence Livermore National Laboratory.
Low power radar circuits are generally capable of detecting the level of one product in a tank at a time. However, frequently, a storage tank contains multiple products layered on top of one another. This creates multiple interfaces between products having different dielectric constants at which the transmitted microwaves can be reflected. For example, in a tank having a water based material and an oil based material, there will typically be two interfaces, one between the air and the oil and one between the oil and the water based material. Sometimes limited intermixing occurs adjacent the product interface. Conventional low power radar level transmitters have been unable to detect both product interfaces, and have thus been unable to detect the level of both products without substantial user supplied dielectric constant data. Further, if the dielectric constants of the process products change, they must be re-entered, or error may be introduced into the system.